The application relates generally to coating of metallic substrates, and more specifically to applying a nondiffused metal coating to an alloy substrate.
Bulk metal coatings have previously been applied directly onto substrates by electrically plating the substrate using an aqueous electrolyte solution, often referred to as electrolytic deposition, electrodeposition, or electroplating. Coatings of more active metals provide protection to the base materials by sacrificial corroding of the coatings, thus have been used extensively. Specifically, an aluminum coating has been shown to be a drop-in replacement of cadmium to protect high strength steel, which is susceptible to hydrogen embrittlement when it is protected by zinc or zinc alloys electroplated in an aqueous plating bath. The only commercial Al electroplating technology in the U.S. is Alumiplate™, which employs a bath that is pyrophoric (triethlyaluminium in toluene) and operates above room temperature (at 100° C.). Such Al electroplating can be difficult and dangerous to implement due in some part to the pyrophoric nature of the plating chemistry and use of organic solvents such as toluene. Toluene is currently listed by the U.S. Environmental Protection Agency (EPA) as a hazardous air pollutant (HAP). The process is also complicated or rendered incompatible for many combinations of coatings and substrates.
Other more advanced coating processes have been developed but each have shortcomings. Thin film chemical vapor deposition (CVD) and physical vapor deposition (PVD) can often be used for more precise control, but cannot be readily scaled up to larger industrial processes, nor is it a practical substitute for bulk deposition of thicker coatings. Recent advances in ionic liquids and related processes have shown promise for depositing metallic coatings directly onto a substrate. Many such coatings are otherwise difficult, dangerous, or impossible to deposit using aqueous electrolytes, e.g. Al, Ti and W coatings. These metals are abundant and excellent for corrosion resistance. However, problems have arisen with ensuring consistent chemical and structural compatibility and adhesion between substrates and the bulk metal layer.